1. Field of the Invention
The present invention relates to an oxygen detecting composition, more particularly, relates to an oxygen detecting composition capable of indicating the presence or absence of oxygen or the degree of oxygen concentration by its color change, and exhibiting a good light and heat stability. The present invention further relates to an oxygen detecting agent and an oxygen detecting ink pigment each comprising the oxygen detecting composition.
2. Description of the Prior Art
Hitherto, there have been proposed oxygen detecting agents utilizing the reversible color change of organic colorants by oxidation-reduction reaction. For example, Japanese Patent Application Laid-Open Nos. 53-117495 and 53-120493 disclose solid oxygen detecting agents comprising an organic colorant such as thiazine dye, azine dye and oxazine dye, a reducing agent and a basic substance. Japanese Patent Application Laid-Open No. 56-84772 discloses an oxygen indicator ink composition prepared by dissolving or dispersing a thiazine dye or the like together with a reducing sugar and an alkaline substance into a resin solution. A commercially available oxygen detecting agent (for example, xe2x80x9cAGELESS EYExe2x80x9d, trademark of Mitsubishi Gas Chemical Company, Inc.) is a functional product for indicating a deoxygenated condition (oxygen content of less than 0.1% by volume) of a transparent packaging container in a simple manner by its color change, and has been used together with an oxygen-absorbing agent (for example, xe2x80x9cAGELESSxe2x80x9d, trademark of Mitsubishi Gas Chemical Company, Inc.) to maintain a freshness of foods and to prevent the degradation of medical and pharmaceutical products.
The conventional oxygen detecting agent, however, is insufficient in the light and heat resistance, for example, tends to cause discoloration or deterioration in color change function upon exposure to light as well as cause browning or deterioration in color change function under high temperatures. To maintain its clear color for a long period of time, therefore, the conventional oxygen detecting agent must be stored by screening from light at low temperatures. This tendency is remarkable in case of a printed oxygen detecting agent comprising an ink having an oxygen detecting function.
In addition, the conventional oxygen detecting agent is opaque to hide the packaged contents such as foods, medical products and pharmaceutical products, thereby unfavorably making the contents invisible outwardly.
An object of the present invention is to provide a light-resistant and heat-resistant oxygen detecting composition and an oxygen detecting agent. Another object of the present invention is to provide a transparent or translucent solid oxygen detecting agent and an oxygen detecting ink pigment.
As a result of extensive research, the inventors have found that an oxygen detecting composition comprising a composite material prepared by mixing a layered silicate (sheet silicate), a cationic surfactant, an organic colorant, a reducing agent and an optional basic substance is excellent in the heat resistance and the light resistance. The inventors have further found that an oxygen detecting agent and an oxygen detecting ink pigment each comprising a composite material of a layered silicate intercalated with a cationic surfactant, an organic colorant, a reducing agent and an optional basic substance is not only excellent in the light resistance and the heat resistance, but also transparent or translucent. The present invention has been accomplished on the basis of these findings.
The oxygen detecting composition according to the present invention contains, as the essential constituting components, a cationic surfactant, an organic colorant, a reducing agent and a layered silicate (sheet silicate), or a cationic surfactant, an organic colorant, a reducing agent, a basic substance and a layered silicate. Preferably, the cationic surfactant, the organic colorant, the reducing agent and the optional basic substance are intercalated into the layered silicate.
The cationic surfactants usable in the present invention are those having cationic atoms and lipophilic groups in their molecules, and being capable of generating organic cations in water by ionization. As the cationic surfactant, a quaternary ammonium salt is typically mentioned, with a quaternary ammonium salt having four carbon groups including at least one lipophilic group bonded to the nitrogen atom being preferred.
The lipophilic group means a non-polar atomic group having a strong affinity for oil, but less interacting with water. Examples of the lipophilic group include chain- and cyclic hydrocarbon groups, aromatic hydrocarbon groups, halogenated alkyl groups, organosilicone groups, and fluorocarbon groups.
The cationic surfactant is preferably cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, stearyltrimethylammonium chloride, stearyltri(methylbenzyl)ammonium chloride, distearyldimethylammonium chloride, or distearyldi(methylbenzyl)ammonium chloride.
The organic colorant usable in the present invention is an aromatic compound having a long conjugated double bond system containing mobile xcfx80 electrons, and is capable of reversibly changing its color by oxidation-reduction reaction. Examples of the organic colorant include oxidation-reduction indicators, thiazine dyes, azine dyes, oxazine dyes, indigoid dyes, and thioindigoid dyes. Specific examples include methylene blue, new methylene blue, methylene green, variamine blue B, diphenylamine, ferroin, capri blue, safranine T, indigo, indigo carmine, indigo white, and indirubin. Of these, preferred are triazine dyes represented by methylene blue.
The reducing agent used in the present invention is a compound capable of reducing the organic colorant in an atmosphere having an oxygen concentration lower than that of atmospheric air. Examples of the reducing agents include monosaccharides such as glucose, fructose and xylose; reducing disaccharides such as maltose; ascorbic acid and its salts; dithionous acid and its salts; and cysteine and its salts.
In some cases, it is preferred to combinedly use the basic substance to enhance the reduction activity of the reducing agent. As the basic substance, there may be used hydroxides such as sodium hydroxide, potassium hydroxide and calcium hydroxide; and carbonates such as sodium carbonate, potassium carbonate and sodium hydrogen carbonate. The amount of the basic substance, if used, is preferably 0.001 to 10 parts by weight, more preferably 0.01 to 1 part by weight based on one part by weight of the layered silicate.
The layered silicate (sheet silicate) used in the present invention has a layered structure that comprises a number of sheets repeatedly stacked in parallel, each sheet being constituted by the atomic groups (inclusive of ioncontaining groups) arranged in plane. Specifically, the layered silicate is an inorganic layered compound that is constructed by tetrahedron sheets composed of silicon atoms, aluminum atoms and oxygen atoms and octahedron sheets composed of aluminum atoms, magnesium atoms, oxygen atoms and hydrogen atoms in a ratio of 1:1 or 2:1.
Further, in addition to the above atoms, the tetrahedron sheet may contain iron atoms, and the octahedron sheet may contain iron atoms, chromium atoms, manganese atoms, nickel atoms or lithium atoms. In addition to molecules of water, cations such as potassium ion, sodium ion or calcium ion may be present as exchangeable cations between the layers of the layered silicate.
The layered silicate used in the present invention is preferably a smectite group silicate, for example, a natural layered silicate such as montmorillonite, beidellite, saponite, hectorite and sauconite which are belonging to smectite group (natural smectite). A layered silicate belonging to smectite group that is produced from an inorganic compound by hydrothermal synthesis (synthetic smectite) is also usable. Of these, preferred is the synthetic smectite.
In the oxygen detecting composition of the present invention, the addition amounts are 0.1 to 100 parts by weight, preferably 0.5 to 50 parts by weight, more preferably 1 to 10 parts by weight for the cationic surfactant, 0.001 to 10 parts by weight, preferably 0.01 to 1 part by weight for the organic colorant, and 0.01 to 200 parts by weight, preferably 0.1 to 100 parts by weight for the reducing agent, each based on one part by weight of the layered silicate.
The oxygen detecting composition of the present invention may be prepared by mixing an aqueous dispersion of the layered silicate and an aqueous solution dissolving the cationic surfactant, the organic colorant, the reducing agent and the optional basic substance.
The oxygen detecting composition of the present invention is preferably a composite material of the layered silicate intercalated with the cationic surfactant, the organic colorant, the reducing agent and the optional basic substance, although not specifically limited thereto. The intercalation of the organic colorant, etc. can be confirmed by X-ray diffraction or by the color change function attributable to the organic colorant and the reducing agent that are brought close together by the action of the intercalated cationic surfactant.
The oxygen detecting composition of the present invention may be made into a powdery oxygen detecting agent by mixing with an inorganic substance. A known inorganic substance such as zeolite may be used, and a basic inorganic substance such as magnesium carbonate is particularly preferable. The amount of the inorganic substance is preferably 50 to 500 parts by weight based on one part by weight of the layered silicate.
The mixture of the oxygen detecting composition of the present invention and the inorganic substance may be tableted into a tablet-type oxygen detecting agent. An oxygen detecting agent of film, sheet or thread shape can be obtained by impregnating a paper, cloth or thread with an alkaline aqueous solution of the oxygen detecting composition of the present invention.
The oxygen detecting agent may be also prepared in the same manner as in the production method for the conventional oxygen detecting agent except for adding the cationic surfactant and using an aqueous dispersion or solution of the solid layered silicate instead of water used in the conventional method.
The intercalation of the cationic surfactant into the layered silicate can be attained by replacing exchangeable cations present between the layers of the layered silicate with the cationic surfactant. Therefore, the cationic surfactant is used in the present invention. If the organic colorant or the reducing agent is cationic, the intercalation thereof can be effected by replacing the exchangeable cations with the organic colorant or the reducing agent. If nonionic, the intercalation of the organic colorant or the reducing agent is effected in cooperation with the cationic surfactant. The intercalation of the basic substance is effected in cooperation with the cationic surfactant.
The interlayer distance of the layered silicate is expanded by the intercalation of the cationic surfactant, organic colorant and reducing agent. The intercalation is therefore confirmed by measuring the interlayer distance, for example, by X-ray diffractometry.
The oxygen detecting composition of the present invention may be produced by mixing an aqueous dispersion of the layered silicate with an aqueous solution containing the cationic surfactant, the organic colorant, the reducing agent and the optional basic substance; removing water by filtration or centrifugation; and drying in the form of a thin film or a massive solid. Alternatively, a solid oxygen detecting composition may be produced by mixing a solid layered silicate with a solid or liquid cationic surfactant, organic colorant, reducing agent and optional basic substance using a mortar or the like. In this method, a small amount of a solvent such as water and alcohol may be added, if required. It was confirmed by X-ray diffractometry that the interlayer distance of the layered silicate contained in the oxygen detecting composition obtained in this method was expanded.
The oxygen detecting composition of the present invention may be a solid pigment and is therefore used as it is or formed into a shaped article such as film. Alternatively, the oxygen detecting composition may be dispersed in another solid material or mixed with another solid material to prepare an oxygen detecting agent in the form of tablet, sheet, film or other shapes. The oxygen detecting composition also serves as a pigment for an oxygen detecting ink, and is therefore made into an oxygen detecting ink by mixing with a solvent, a binder or the like. Letters, figures, patterns, etc. of the oxygen detecting ink can be provided on paper, plastic tape, etc. by the application or printing, thereby obtaining the oxygen detector for indicating the presence (or absence) of oxygen. In addition, by printing letters, figures, patterns, etc. with the oxygen detecting ink on the inner surface of a gas-barrier container, the surface of an oxygen absorbing package, etc., the presence or absence of oxygen within the container can be recognized from outside.
The present invention will be described in more detail below with reference to the following examples and comparative examples. However, these examples are only illustrative and not intended to limit the present invention thereto.